Flux and method of producing same



Patented Dec. 30, 1941 2,267,763 FLUX AND METHOD or rnonoomc SAME JohannS. Streicher, Newark, N. J., assignor to The American Platinum Works,Newark, N. J., a corporation or New Jersey No Drawing. Originalapplication November 8,

1940, Serial No. 364,794. Divided and this application May 3, 1941,Serial No. 391,714

2 Claims.

This invention relates to a flux for use primarily in connection withsoldering operations involving soldering with hard metal solder.

This application is a division of my co-pending application Serial No.364,794 filed November 8, 1940, which in turn is a continuation in partof my earlier application Serial No. 250,931 filed January 14, 1939.

surfaces over which the liquid solder is to spread and may also beapplied to the solder itself.

Hard metal solders for which the present flux is primarily intendedcomprise such solders as silver or brass solders and are used forinstance in the soldering of such metals as silver, copper, iron, nickeland various alloys containing one or more such metals including, forinstance, stainless steel.

The common fluxes generally used for silver solders comprise borax orboric acid or mixtures thereof. Such fluxes when heated are generallytransformed into viscous masses having no great liquidity, and due totheir great surface tension they offer a great resistance to the flow ofthe liquid solder. Such fluxes when heated tend to develop gas bubblesand do not have a definite and predetermined melting point but meltgradually like glass irrespective of whethter or not suchfluxes alsocontain a small amount of modifying agents. Boric acid for instance isgradually transformed into a viscous liquid, beginning to thicken at 871C. and increasing in viscosity with decreasing temperatures until itbecomes solid at 565 C. Also, such fluxes are not suf-' ficientlyeffective in the production of perfect solder joints with such metals,for instance stainless steel, which form or carry surface oxide layersdifficult to remove.

It has bcen'the general belief in connection with solde ing operationsinvolving the use of hard metal solders that any protective fluxes,particularly a viscous or glassy flux, will be satisfactory so long asthe right solder is used, with the result that in such solderingoperations it has been practice to use a single flux or a limited numberof fluxes and to use a great number of hard metal solders, particularlysilver solders, a

new solder having usually been produced for eV- r ery apparentlydifferent, but in reality similar, problem. It is one object of myinvention to provide a flux which when heated has a low surface tensionand is highly liquid, thus permitting the free and unimpeded flow ofhard metal solders, particularly of silver solders. It is another objectof my invention to provide a flux having a deflnite melting point and toprovide a series of fluxes similar in composition to each other meltingat different but definite melting points, each such flux having a lowsurface tension. Other ob- J'ects and advantages of my flux will beapparent from the description hereinafter following.

The flux according to my invention is produced by reacting a mixture ofboric acid and sodium fluoride in water in certain specifiedproportions.

The reaction takes place spontaneously at room temperature if themixture is subjected to a thorough grinding procedure. The reaction alsotakes place without the necessity of thorough grinding when the mixtureis heated to a temperature of about C. to C. and more, whereafter thereacted mixture is cooled. The reaction is exothermic, evolvingconsiderable heat, and produces a hydrated polyboric acid compound,probably tetraboric acid and pentaboric acid, containing water as aconstituent thereof. The compound also seems to contain sodium fluorideas a constituent thereof. When heated the reacted mixture quickly losesany excess of water, but the water which is contained in the boric acidcompound as a constituent thereof is released only slowly and graduallyand only in. part.

The reaction compound melts at a definite melting point and when moltenhas a surprisingly low surface tension. The molten mass, when applied toor produced on metal surfaces, has a considerably greater cleaningcapacity than other fluxes comprising boric acid, borax, or mixturesthereof, as the complex boric acid compound thus produced develops, whenheated and fused, a strong acidity, contrary to boric acid which whenheated and fused is transformed into amorphous boric oxide which is aglassy substance having in the fused state no or little acid character.

The flux comprising the boric acid compound described makes it possibleto effect soldering operations with hard metal solder with greater easeand better technical performance.

The melting points of the series offluxes according to my invention arelow and correspond to the melting points of the commonly used sil- 1individual melting points are produced from mix tures of 45% to 29%sodium fluoride and 55% to 11% boric acid, such fluxes having melting'points within a range from 843 C. to 1715 C.

The hydrated complex polyboric acid fluxes are best produced by firstmixing together, preferably in a mortar or grinding machine, the sodiumfluoride and boric acid in the specified proportions and then addingtheproper amount of water.

Since boric acid does not readily wet water, there may be added to thewater a trace of wetting agent. The liquid crystallizes while evolvingheat to result in a hard mass which is then ground into a thin paste.The flux may also be produced by dissolving sodium fluoride in hot waterand then adding boricv acid, the hydrated complex polyboric acidcompound crystallizing after cooling. For instance, a mixture ofcommercial boric acid, dehydrated commercial sodium fluoride, and watercontaining a trace of Lamepon A as wetting agent, is finely ground untilthe mass is transformed into 'a thick gray liquid which on standingturns itself by crystallization into a white mass which is thenthoroughly ground and thereby transformed into a smooth pasteconstituting the flux.

The hydrated flux forms a very stable base and can be kept in such statefor an indefinite length of time as distinguished from other fluxeswhich tend to cake and solidify in their jars after a relatively shorttime..

The flux can be transformed into dry flux powder by heating thecrystallized acid compound either as such or in the form of the hydratedflux paste at about 110 C. to"volatilize the excess of water otherwisenecessary for keeping the hydrated flux in the state of a stable pasteand by then comminuting the dehydrated or partly dehydrated solid massinto a fine powder. A verygood non-rising flux can be obtained by fusingthe flux in hydrated state and grinding the resulting crystal cakesinto' fine powder. The

dehydrated fluxes are highly hygroscopic.

' point makes it possible to perform soldering operations with hardmetal solder in such manner that the flux is selected for the desiredsolder instead of using a special solder for the flux used. By thusvarying the flux, it is possible to decrease the number of hard metalsolders to a minimum. Many soldering problems can thus be solvedprimarily by .the selection of 'a flux having the proper physicalproperties, rather than by resorting to selecting a special solder froma complicated and great number of hard metal solders. Also, by thusdivorcing the soldering operation from the great number and variety ofsolders and putting the emphasis on the selection of a proper flux, itis possible to give proper consideration to other qualities of solderswhich it was heretofore not always possible to consider in the selectionof a proper solder. Thus, the color of the solder can be such asto'apprommate the color of the metal to be soldered, or the silversmithcan use a silver solder which does not change the silver content of thearticle to be soldered. Also, some solders will diffuse more readilythan others with the metals to be solthe hard metal solder with which itis used. While the fluxes according to my invention can be used in anydesired manner, by applying them to the metal to be soldered or applyingthem to the solder itself, I have found that they are also exceptionallyadapted in connection with salt bath furnaces recently introduced forsilver brazing operations, since they remain in an active state-for longperiods of time.

Thus my invention provides an exceptionally advantageous flux of greatliquidity and low surface tension of definite and low melting point ofgreat cleaning capacity and free from undesirable shortcomings. The fluxspreads easily and freely over the metal surface to which it is appliedand thus permits the hard metal solder to also spread easily over themetal surface. It makes possible perfect bonds in many cases where otherfluxes are unsatisfactory. By providing a series of such fluxes eachhaving a different but definite melting point, it is possible to selecta suitable flux for any hard metal solder, thus decreasing the number ofhard metal solders and permitting the selection of such solders forspecial properties not connected with requirements imposed by theparticular flux used. The flux is not poisonous in use and its stabilityand effectiveness are such that one and the same flux can be used formany successive soldering operations without losing its effectiveness.

The fluxes according to my invention consist essentially of the reactionproduct produced in a mixture of boric acid and sodium fluoride insuitable proportions, and may contain water as when it is in thehydrated state.- The reaction mixture and the flux itself should besubstantial- 1y free from other substances, but may contain "smallquantities of other substances so long as the fundamentalcharacteristics of my polyboric acid compound'flux are not substantiallyaltered in a detrimental manner.

What I claim is:

1. The method of producing a flux for hard metal solder, comprisingreacting an aqueous mixture of boric acid and sodium fluoride in theproportions of fifty-five (55%) per cent to seventy-one (71%) per centboric acid and fortyflve (45%) per cent to twenty-nine (29%) per centsodium fluoride.

2. A flux for hard metal solder, comprising a polyboric acid reactionproduct of an aqueous mixture of fifty-five (55%) per cent to seventyone(71%) per cent boric acid and forty-five (45%) per cent to twenty-nine(29%) per cent sodium fluoride.

JOHANN S. STREICIER.

